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| United States Patent |
5,268,023
|
|
Kirner
|
December 7, 1993
|
Nitrogen adsorption with highly Li exchanged X-zeolites with low Si/Al
ratio
Abstract
Low silica X (LSX) zeolites having a framework Si/Al ratio equal to 1.0
with lithium exchange levels greater than a threshold level of 70% exhibit
unexpectedly higher capacity for nitrogen adsorption compared to
LSX-zeolite with lower lithium exchange levels. These materials provide
high performance adsorbents for PSA air separation processes at a lower
cost for the adsorbent because of the lower threshold lithium exchange
levels compared to highly exchanged lithium X-zeolite known in the prior
art.
| Inventors:
|
Kirner; John F. (Orefield, PA)
|
| Assignee:
|
Air Products and Chemicals, Inc. (Allentown, PA)
|
| Appl. No.:
|
957532 |
| Filed:
|
October 5, 1992 |
| Current U.S. Class: |
95/103; 95/130 |
| Intern'l Class: |
B01D 053/04 |
| Field of Search: |
55/25,26,58,62,68,75,389
|
References Cited
U.S. Patent Documents
| 3140931 | Jul., 1964 | McRobbie | 55/25.
|
| 3140933 | Jul., 1964 | McKee | 55/68.
|
| 3313091 | Apr., 1967 | Berlin | 55/58.
|
| 4013429 | Mar., 1977 | Sircar et al. | 55/33.
|
| 4606899 | Aug., 1986 | Butter et al. | 423/328.
|
| 4810265 | Mar., 1989 | Lagree et al. | 55/75.
|
| 4859217 | Aug., 1989 | Chao | 55/68.
|
| 4925460 | May., 1990 | Coe et al. | 55/75.
|
| 4964889 | Oct., 1990 | Chao | 55/75.
|
| 5152813 | Oct., 1992 | Coe et al. | 55/75.
|
| 5171333 | Dec., 1992 | Maurer | 55/75.
|
| 5174979 | Dec., 1992 | Chao et al. | 55/58.
|
| Foreign Patent Documents |
| 1580928 | Dec., 1980 | GB.
| |
Other References
Sherry, H. S.; J. Phys. Chem; 1966, 70, 1158; "The Ion-Exchange Properties
Zeolites. I. Univalent Ion Exchange in Syn. Faujasite".
Kuhl, G. H.; "Crystallization of Low-Silica Faujasite"; Zeolites; 1987, 1,
451.
Lechert, H. et al.; "Investigations on the Crystallization of X-Type
Zeolites"; Zeolites; 1991, 11, 720.
Smith, O. J. et al.; "The Optimal Design of Pressure Swing Adsorption
Systems"; Chem. Eng. Sci., 1991, 46(12), 2967-2976.
Meyers, A. L. et al.; "Thermodynamics of Mixed-Gas Adsorption"; Am. Inst.
of Chem. Eng. J.; 1965, 11, 121.
McCabe, W. L. et al.; "Unit Operations of Chemical Engineering"; 3rd Ed;
McGraw Hill New York, 1976, p. 534.
Miller, G. W.; "Equilibria of Nitrogen, Oxygen, Argon, and Air in Molecular
Sieve SA"; Am. Inst. of Chem. Eng. J.; 1987, 33, 194.
|
Primary Examiner: Spitzer; Robert
Attorney, Agent or Firm: Chase; Geoffrey L., Simmons; James C., Marsh; William F.
Claims
I claim
1. A process for adsorbing nitrogen from a gas containing nitrogen which
comprises contacting the gas with an adsorbent which is selective for the
adsorption of nitrogen, comprising a crystalline X-zeolite having a
zeolitic Si/Al ratio less than 1.25 and a lithium ion exchange level of
the exchangeable ion content greater than 70% and less than 88%.
2. The process of claim 1 wherein said zeolite is ion exchanged with
lithium to a level greater than (72.times.(Si/Al ratio)-2)%.
3. The process of claim 1 wherein said zeolite is ion exchanged with
lithium to a level greater than 77%, but less than 88%.
4. The process of claim 3 wherein said zeolite is ion exchanged with
lithium to a level of approximately 85%.
5. The process of claim 1 wherein said Si/Al ratio is less than 1.1
6. The process of claim 5 wherein said Si/Al ratio is approximately 1.
7. The process of claim 1 wherein said gas contains nitrogen and oxygen.
8. The process of claim 7 wherein said gas is air.
9. The process of claim 1 wherein an oxygen and nitrogen containing gas
contacts a zone of said adsorbent, the nitrogen is selectively adsorbed
and the oxygen passes through said zone and is recovered as an oxygen
enriched product.
10. The process of claim 9 wherein said oxygen enriched product has a
purity of at least approximately 90% oxygen.
11. The process of claim 9 wherein the adsorption is conducted at an
average bed temperature in the range of approximately 55.degree. to
135.degree. F.
12. The process of claim 9 wherein said zone is operated through a series
of steps comprising: adsorption, during which said gas contacts said
adsorbent, nitrogen is selectively adsorbed and oxygen passes through said
zone as product; depressurization, during which said gas contact is
discontinued and said zone is reduced in pressure to desorb the nitrogen;
and repressurization with oxygen product to adsorption pressure.
13. The process of claim 1 wherein the adsorption pressure is in the range
of approximately 35 to 65 psia.
14. The process of claim 1 wherein the depressurization is conducted down
to a level in the range of approximately 14.7 to 16.7 psia.
15. The process of claim 1 wherein said zone is operated through a series
of steps comprising: adsorption, during which said gas contacts said
adsorbent, nitrogen is selectively adsorbed and oxygen passes through said
zone as product; depressurization, during which said gas contact is
discontinued and said zone is reduced in pressure to desorb the nitrogen;
evacuation to further desorb the nitrogen to below ambient pressure; and
repressurization with oxygen product to the adsorption pressure.
16. The process of claim 15 wherein the adsorption pressure is in the range
of approximately 900 to 1600 torr.
17. The process of claim 15 wherein the evacuation is conducted down to a
level in the range of approximately 80 to 400 torr.
Description
TECHNICAL FIELD
The present invention is directed to gas separations using nitrogen
selective adsorbents. More particularly, the present invention is directed
to lithium exchanged X-zeolites with low Si/Al ratios to recover oxygen or
nitrogen from gas mixtures containing them, such as air. Additionally, an
objective is a reduced level of lithium in order to reduce the cost of the
adsorbent.
BACKGROUND OF THE PRIOR ART
Adsorptive separations using zeolitic structures as adsorbents are well
known in the prior art for resolving a multitude of gas mixtures. Such
separations are predicated upon the compositions of the gas mixtures and
the components' selectivity for adsorption on adsorbents, such as
zeolites.
The use of nitrogen in industrial gas applications has seen significant
growth, particularly with the development of non-cryogenic gas mixture
separations. A major field of nitrogen separation comprises the separation
of nitrogen from air. The removal of nitrogen from air results in an
enriched oxygen gas component which is less strongly adsorbed by
appropriate zeolites which are selective for nitrogen adsorption. When
oxygen is desired as product typically at elevated pressure, it is
desirable to adsorb nitrogen from air to result in unadsorbed oxygen
enriched product passing over a nitrogen selective adsorbent. The nitrogen
is then removed during a stage of desorption, typically at lower pressure.
This results in oxygen being recovered at the pressure of the feed air,
while nitrogen is recovered at a pressure below the feed air pressure. As
a result, for the production of oxygen without significant pressure loss
in an adsorptive separation of air, it is desirable to utilize nitrogen
selective adsorbents, such as the family of zeolites.
Although various zeolites are naturally occurring and various synthetic
zeolites are known, some of which have appropriate selectivities for
nitrogen over oxygen and other less strongly adsorbed substances such as
hydrogen, argon, helium, and neon, the industry has attempted to enhance
the performance of various zeolites to improve their selectivity and
capacity for nitrogen over such less strongly adsorbed substances such as
oxygen.
For instance, U.S. Pat. No. 3,140,931 claims the use of crystalline
zeolitic molecular sieve material having apparent pore sizes of at least
4.6 Angstroms for separating oxygen-nitrogen mixtures at subambient
temperatures. U.S. Pat. No. 3,140,933 claims the use of lithium X-zeolite
to separate oxygen-nitrogen mixtures at feed pressures between 0.5 and 5
atm and at a temperature between about 30.degree. C. and -150.degree. C.
U.S. Pat. No. 4,859,217 claims a process for selectively adsorbing
nitrogen using X-zeolite having a framework Si/Al molar ratio not greater
than 1.5 and having at least 88% of its AlO.sub.2 tetrahedral units
associated with lithium cations. This invention is based on the discovery
that nitrogen adsorption from lithium X-zeolite at very high levels of
lithium exchange is not predictable from the trend of the data obtained
for samples with less than 86 equivalent percent lithium exchange and the
remainder principally sodium.
Despite the performance of very high lithium exchange levels of X-zeolite
for air separation, lower exchange levels of lithium would be desirable
because it is costly to manufacture highly lithium exchanged materials.
Large amounts of expensive lithium salts are required to prepare the
highly lithium exchanged forms from the as-synthesized sodium form because
the ion exchange of lithium for sodium is thermodynamically unfavorable.
The present invention overcomes the drawbacks of synthesizing high lithium
exchange while still providing good performance as will be set forth in
greater detail below.
BRIEF SUMMARY OF THE INVENTION
The present invention is a process for selectively adsorbing nitrogen from
a gas mixture containing nitrogen and at least one less strongly adsorbed
component which comprises contacting the gas mixture with an adsorbent
which is selective for the adsorption of nitrogen, comprising a
crystalline X-zeolite having a zeolitic Si/Al ratio less than 1.25 and a
lithium ion exchange level of the exchangeable ion content greater than
70% and less than 88%.
Preferably, the zeolite is ion exchanged with lithium to a level greater
than (72.times.(Si/Al ratio)-2)%.
Preferably, the zeolite is ion exchanged with lithium to a level greater
than 77%, but less than 88%.
Preferably, the zeolite is ion exchanged with lithium to a level of 85%.
Preferably, the Si/Al ratio is less than 1.1.
More preferably, the Si/Al ratio is approximately 1.
Preferably, the gas mixture contains nitrogen and oxygen.
More preferably, the gas mixture is air.
Preferably, an oxygen and nitrogen containing gas mixture contacts a zone
of such adsorbent, the nitrogen is selectively adsorbed and the oxygen
passes through the zone and is recovered as an oxygen enriched product.
Preferably, the oxygen product has a purity of at least approximately 90%
oxygen.
Preferably, the adsorption is conducted at an average bed temperature in
the range of approximately 55.degree. to 135.degree. F.
Preferably, the zone is operated through a series of steps comprising:
adsorption, during which the gas mixture contacts the adsorbent, nitrogen
is selectively adsorbed and oxygen passes through the zone as product;
depressurization during which the gas mixture contact is discontinued and
the zone is reduced in pressure to desorb the nitrogen; and
repressurization with oxygen product to the adsorption pressure.
Preferably, the adsorption pressure is in the range of approximately 35 to
65 psia.
Preferably, the depressurization is conducted down to a level in the range
of approximately 14.7 to 16.7 psia.
Alternatively, the zone is operated through a series of steps comprising:
adsorption, during which the gas mixture contacts the adsorbent, nitrogen
is selectively adsorbed and oxygen passes through the zone as product;
depressurization during which the gas mixture contact is discontinued and
the zone is reduced in pressure to desorb the nitrogen; evacuation to
further desorb the nitrogen to below ambient pressure; and
repressurization with oxygen product to the adsorption pressure.
Preferably, the adsorption pressure of this alternative is in the range of
approximately 900 to 1600 torr.
Preferably, the evacuation is conducted down to a level in the range of
approximately 80 to 400 torr.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph of N.sub.2 capacity at 0.9 atm, 23.degree. C. as a
function of lithium exchange level for powder lithium, sodium LSX-zeolite
(Si/Al=1.0) observed in the present invention and for binderless lithium,
sodium X-zeolite (Si/Al=1.25) disclosed in U.S. Pat. No. 4,859,217.
FIG. 2 is a graph of binary N.sub.2 /O.sub.2 selectivity calculated by IAST
for air feed at 1.45 atmospheres at 30.degree. C. as a function of lithium
exchange level for lithium, sodium LSX-zeolite (Si/Al=1.0) observed in the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
The process of the present invention, directed to nitrogen adsorption from
gas mixtures of less strongly adsorbed components, such as oxygen,
hydrogen, argon, and helium, is achieved by the use of a crystalline
X-zeolite having a zeolitic Si/Al ratio less than 1.25 and a lithium ion
exchange level of the exchangeable ion content greater than 70% and less
than 88%. The threshold lithium exchange level required to observe the
improved performance of the present invention varies with Si/Al ratio, and
can be described by the equation:
threshold lithium exchange level=(72.times.(Si/Al)-2)%
Preferably, the X-zeolite is a low silica X-zeolite (LSX-zeolite) having a
framework Si/Al ratio of approximately 1 and with a lithium exchange level
greater than 77%, but less than 88%.
Lithium, sodium LSX-zeolites having a framework Si/Al ratio equal to 1.0
with lithium exchange levels greater than a threshold level of 70% exhibit
unexpectedly higher capacity for nitrogen adsorption compared to
LSX-zeolite with lower lithium exchange levels. The threshold lithium
exchange level for improved performance over the pure sodium form is
unexpectedly lower for lithium, sodium LSX-zeolite (Si/Al=1.0) than for
lithium, sodium X-zeolite (Si/Al=1.25). Furthermore, for any lithium
exchange level greater than 70%, the nitrogen capacity of lithium, sodium
LSX-zeolite is always higher than that for lithium, sodium X-zeolite at
the same lithium exchange level.
These materials provide high performance adsorbents for PSA air separation
processes at a lower cost for the adsorbent because of the lower threshold
lithium exchange levels compared to highly exchanged lithium X-zeolite
known in the prior art.
A variety of synthesis processes are available to prepare the preferred low
silica X-zeolite. In U.K. Patent 1,580,928, a process for making low
silica X-zeolites is set forth comprising preparing an aqueous mixture of
sources of sodium, potassium, aluminate, and silicate, and crystallizing
the mixture at below 50.degree. C. or aging the mixture at 50.degree. C.
or below followed by crystallizing the same at a temperature in the range
of 60.degree. C. to 100.degree. C.
Gunter H. Kuhl in an article, "Crystallization of Low-Silica Faujasite,"
appearing in Zeolites 1987, 7, 451, disclosed a process for making low
silica X-zeolites comprising dissolving sodium aluminate in water with the
addition of NAOH and KOH. Sodium silicate was diluted with the remaining
water and rapidly added to the NaAlO.sub.2 --NaOH--KOH solution. The
gelled mixture was then aged in a sealed plastic jar for a specified time
at a specified temperature. The product was filtered and washed.
In U.S. Pat. No. 4,606,899, a process for preparing Maximum Aluminum
X-zeolite from clay is set forth in which kaolin clay, calcined to at
least 700.degree. C. is converted to LSX-zeolite by agitating a reaction
mixture, comprised of the clay with sodium and potassium hydroxide, at
temperatures in excess of 50.degree. C. and seeding the resulting mixture
with LSX-zeolite at a predetermined time after the reaction has been
initiated.
Other processes are available for preparing X-zeolite with Si/Al ratios
intermediate between than of LSX-zeolite (Si/Al=1.0) and conventional
X-zeolite (Si/Al 1.25) such as that described in Lechert, H. and Kacirek,
H. Zeolites 1991, 11, 720, which shows that the Si/Al ratio of
crystallizing X zeolites depends distinctly on the alkalinity of the batch
but only weakly on its Si/Al ratio.
Although other ion forms of X-zeolites with Si/Al ratios less than 1.25 can
be used, typically the as-synthesized sodium or mixed sodium, potassium
X-zeolite is used to prepare the lithium exchanged zeolite using ion
exchange procedures well known in the art. Typically the ion exchange is
accomplished by contacting the sodium or mixed sodium, potassium X-zeolite
with an aqueous solution of a lithium salt. Because the exchange is
thermodynamically unfavorable, multiple stages would normally be required.
Alternatively, the ion exchange can be performed by contacting the zeolite
with lithium solution in a column after granulating the assynthesized form
of the zeolite. Other methods of ion exchange are contemplated and can be
used for the present invention.
For use in industrial gas separation processes, the zeolite is generally
used in granulated form. A variety of processes are known in the art using
binders, such as clays, silica, alumina, and the like, and granulation
equipment, such as extruders, disk or roll granulators, and the like.
The adsorbent must be dehydrated before being used for gas separation using
a thermal activation step. Such a thermal activation step can be achieved
by a number of different methods in which the zeolitic water and the
hydration spheres are carefully removed and the amount of water in the
gaseous environment in contact with the zeolite during this step is
minimized.
A preferred use for the lithium exchanged X-zeolites of the present
invention is the separation of nitrogen from oxygen in air using a
pressure swing adsorption (PSA) or vacuum swing adsorption (VSA) process.
In such a process, an adsorbent bed comprising lithium X-zeolite, as
described above, is initially pressurized with oxygen. A gas stream
comprising nitrogen and oxygen, such as air, at a temperature between
0.degree. and 50.degree. C. and a pressure between 1 atmosphere and 5
atmospheres, is passed over the adsorbent bed. A portion of the nitrogen
in the gas stream is adsorbed by said lithium exchanged X-zeolite, thereby
producing an oxygen-enriched product stream. The nitrogen containing
adsorbent bed is subsequently depressurized and evacuated with the option
of being purged with oxygen enriched gas to produce a nitrogen enriched
stream. The bed is then repressurized with product oxygen and adsorption
can be reinitiated.
Alternatively, these materials can be used for recovering a nitrogen
enriched product using, for example, an existing nitrogen vacuum swing
adsorption process as described in U.S. Pat. No. 4,013,429, wherein the
process includes the steps of feed, rinse, desorption, and
repressurization wherein the nitrogen enriched product is recovered as the
absorbate during desorption.
The zeolitic adsorbents used in demonstrating the invention were prepared
in the following way.
Sodium, potassium LSX-zeolite was prepared by the method of Kuhl
("Crystallization of Low-Silica Faujasite" in Zeolites 1987, 7, 451) which
comprises dissolving sodium aluminate in water with the addition of NaOH
and KOH. Sodium silicate is diluted with the remaining water and rapidly
added to the NaAlO.sub.2 --NaOH--KOH solution. The gelled mixture is then
aged in a sealed plastic jar for a specified time at a specified
temperature. The product is filtered and washed.
Lithium LSX-Zeolite was prepared by ion exchange of sodium, potassium
LSX-zeolite powder using five static exchanges at 100.degree. C. with a
6.3-fold equivalent excess of 2.2 M LiCl. Sodium LSX-zeolite was prepared
by ion exchange of sodium, potassium LSX-zeolite using three static
exchanges at 100.degree. C. with a 4.2-fold equivalent excess of 1.1 M
NaCl. Various exchange levels of lithium, sodium LSX-zeolite were prepared
by adding separate samples of the initially prepared lithium LSX-zeolite
powder to appropriate amounts of 0.1 M NaCl and stirring at room
temperature for about 4 hours. The mixed cation samples were filtered but
not washed to prevent hydrolysis of the lithium cations. The use of dilute
solution made the errors in cation levels introduced by the solution
retained on the filter cake insignificant.
The samples were analyzed by Inductively Coupled Plasma-Atomic Emission
Spectroscopy (ICP-AES) for silicon and aluminum and Atomic Absorption
Spectroscopy for lithium, sodium, and potassium. Table I contains the
results of elemental analyses for lithium and sodium in the exchanged
samples.
TABLE 1
__________________________________________________________________________
Nitrogen Capacity and N.sub.2 /O.sub.2 Selectivity for Lithium, Sodium
LSX-Zeolite
sample
Li/Al
Na/Al
N.sub.m (obs),.sup.a mmol/g
N.sub.m (delta),.sup.b
number
eq ratio
eq ratio
0.9 atm
1.0 atm
mmol/g
.alpha. (N.sub.2 O.sub.2).sup.c
__________________________________________________________________________
1 1.03 0.01 1.28 1.35 0.90 10.0
2 0.90 0.10 1.01 1.06 0.70 n/a
3 0.83 0.20 0.70 0.74 0.51 5.7
4 0.70 0.27 0.44 0.47 0.32 4.0
5 0.64 0.34 0.38 0.40 0.28 n/a
6 0.58 0.45 0.40 0.42 0.29 n/a
7 0.43 0.55 0.40 0.42 0.29 n/a
8 0.30 0.66 0.36 0.39 0.26 n/a
9 0.21 0.75 0.37 0.39 0.26 n/a
10 0.11 0.86 0.41 0.44 0.31 n/a
11 n/a 1.00 0.40 0.43 0.30 3.6
__________________________________________________________________________
.sup.a N.sub.m (obs) = nitrogen capacity at 23.degree. C. at indicated
pressure.
.sup.b N.sub.m (delta) = isothermal nitrogen working capacity from 0.2 to
1.0 atm at 23.degree. C.
.sup.c .alpha. (N.sub.2 /O.sub.2) = N.sub.2 /O.sub.2 selectivity for air
at 1.45 atm, 30.degree. C., calculated from IAST.
n/a = not analyzed
Adsorptive capacities for nitrogen (N.sub.2) were obtained using a
conventional McBain gravimetric adsorption unit. Samples were first
superficially dried at 110.degree. C. in an oven purged with N.sub.2 at a
high flow rate. Approximately 5 g were loaded into the McBain sample
buckets, and the samples were heated under vacuum at 1.degree. C./min or
less to 550.degree. C. The samples were held at 550.degree. C. until the
pressure dropped to about 10 microns of Hg. After activation, N.sub.2
isotherms were obtained to 1 atm at 23.degree. C. The isotherm data was
fit to the Langmuir expression:
N.sub.m =mbP/(1+Bp)
where N.sub.m is the amount adsorbed, P is the pressure, m is the monolayer
capacity, and b is the affinity parameter. The fits were used to generate
N.sub.2 capacities and isothermal N.sub.2 working capacities (reported in
Table I).
FIG. 1 compares N.sub.2 capacity at 0.9 atm, 23.degree. C. as a function of
lithium exchange level in lithium, sodium LSX-zeolite (Si/Al=1.0) to
N.sub.2 capacity at 700 torr, 23.degree. C. for lithium, sodium X-zeolite
(Si/Al=1.25) disclosed in U.S. Pat. No. 4,859,217. For lithium, sodium
LSX-zeolite with lithium exchange levels of 64% and lower, N.sub.2
capacity is indistinguishable from the N.sub.2 capacity of the 100% sodium
form, with an N.sub.2 capacity of about 0.39 mmol/g. For lithium, sodium
LSX-zeolite with lithium exchange levels from about 70% to 100%, N.sub.2
capacity increases nearly linearly with increasing lithium exchange level.
The threshold lithium exchange level for improved performance over the
pure sodium form is unexpectedly lower for lithium, sodium LSX-zeolite
than for lithium, sodium X-zeolite. Furthermore, for any lithium exchange
level greater than 70%, the N.sub.2 capacity of lithium, sodium
LSX-zeolite is always higher than that for lithium, sodium X-zeolite at
the same lithium exchange level.
The N.sub.2 capacities of lithium, sodium LSX-zeolites with lithium
exchange levels above about 70% could not have been predicted based on the
most relevant prior art of U.S. Pat. No. 3,140,933 and U.S. Pat. No.
4,859,217. Although U.S. Pat. No. 3,140,933 teaches the benefits of
lithium exchanged X-zeolites, it does not indicate any further benefits to
be gained by using X-zeolite with Si/Al ratios lower than 1.25. In the
present invention, it has been found unexpectedly that the N.sub.2
capacity of LSX-zeolite (Si/Al=1.0) is always higher than the N.sub.2
capacity of X-zeolite (Si/Al=1.25) at the same lithium exchange level,
provided that the lithium exchange level is above the threshold level of
70% Li exchange. As a specific example, the N.sub.2 capacity of 0.87
mmol/g for 86% lithium LSX-zeolite is substantially higher than the
N.sub.2 capacity of 0.56 mmol/g for 86% lithium X-zeolite, the lithium
exchange level specifically disclosed for lithium X-zeolite in U.S. Pat.
No. 3,140,933. (The N.sub.2 capacities for 86% lithium exchange were
obtained from straight line fits to the high lithium exchange data in FIG.
1 for LSX-zeolite and X-zeolite.)
U.S. Pat. No. 4,859,217, on the other hand, did recognize the outstanding
performance of lithium LSX-zeolite (Si/Al=1.0) over lithium X-zeolite
(Si/Al=1.25), but only at essentially 100% lithium exchange. It does not
suggest that the threshold lithium exchange level to demonstrate improved
performance of the lithium, sodium form over the pure sodium form would be
lower for LSX-zeolite than for X-zeolite. In the present invention it has
been found unexpectedly that the threshold for enhanced nitrogen capacity
of 70% lithium exchange for LSX-zeolite with Si/Al=1.0 is substantially
lower than the threshold of 88% claimed in U.S. Pat. No. 4,859,217 for
X-zeolite with Si/Al<1.5.
N.sub.2 capacities of lithium, sodium LSX-zeolites with lithium exchange
levels above about 77% are even more unexpected than the N.sub.2
capacities of those with lithium exchange levels above about 70% compared
to the most relevant prior art of U.S. Pat. No. 3,140,933 and U.S. Pat.
No. 4,859,217. The N.sub.2 capacity of 0.62 mmol/g observed for 77%
lithium LSX-zeolite is significantly higher than the N.sub.2 capacity of
0.56 mmol/g observed for the 86% lithium X-zeolite disclosed in U.S. Pat.
No. 3,140,933 and about the same as the N.sub.2 capacity of 0.62 mmol/g
observed for the threshold exchange level of 88% lithium X-zeolite in U.S.
Pat. No. 4,859,217.
The threshold lithium exchange level of 70% observed for lithium, sodium
LSX-zeolite (Si/Al=1.0) can be combined with the threshold lithium
exchange level of 88% disclosed for lithium, sodium X-zeolite (Si/Al=1.25)
in U.S. Pat. No. 4,859,217 to predict threshold lithium exchange levels
for X-zeolites with Si/Al ratios intermediate between 1.0 and 1.25:
threshold lithium exchange level=(72.times.(Si/Al)-2)%
N.sub.2 capacity alone is not a measure of an adsorbent's ability to effect
a separation of N.sub.2 from other components. Berlin in U.S. Pat. No.
3,313,091 points out the importance of the shape and slope of the
component isotherms in the pressure region of interest. Consequently, the
isothermal N.sub.2 working capacities from 0.2 to 1.0 atm, a pressure
region of interest for O.sub.2 VSA air separation processes, were also
determined from the isotherm fits and are included in Table I. The
adsorbents of the present invention also show high isothermal N.sub.2
working capacities which are very important for PSA N.sub.2 processes.
An additional property required of nitrogen adsorbents is high selectivity
for adsorption of nitrogen over the less strongly adsorbed components of
the gas mixture to be separated. For example, the binary N.sub.2 /O.sub.2
selectivity at feed pressure is an indicator of the recovery losses from
oxygen coadsorbed with nitrogen on the adsorbent bed in oxygen VSA air
separation processes.
N.sub.2 and O.sub.2 isotherms were obtained for several of the lithium,
sodium LSX-zeolite samples using a high pressure volumetric isotherm unit.
Approximately 2-2.5 g of sample was loaded into a stainless steel sample
cylinder protected with a 20-micron filter to prevent loss of sample. The
samples were heated under vacuum at 1.degree. C./min or less to
400.degree. C. and held at 400.degree. C. until the pressure dropped below
1.times.10.sup.-5 torr. After activation, N.sub.2 and O.sub.2 isotherms
were obtained to 12,000 torr at 23.degree. and 45.degree. C. The isotherm
data was fit to standard adsorption isotherms. Binary N.sub.2 /O.sub.2
selectivities were calculated using ideal adsorbed solution theory (IAST)
for air feed at 1.45 atmospheres, 30.degree. C., where N.sub.2 /O.sub.2
selectivity is defined as:
##EQU1##
where N.sub.N2 =N.sub.2 coadsorbed at N.sub.2 partial pressure in the feed
N.sub.O2 =O.sub.2 coadsorbed at O.sub.2 partial pressure in the feed
Y.sub.N2 =mole fraction of N.sub.2 in the feed
Y.sub.O2 =mole fraction of O.sub.2 in the feed
The binary N.sub.2 /O.sub.2 selectivities are also included in Table I. The
adsorbents of the present invention also show high N.sub.2 /O.sub.2
selectivity. The threshold lithium exchange level for observing an
improvement in the N.sub.2 /O.sub.2 selectivity of lithium, sodium
LSX-zeolite over that of the pure sodium form is also about 70% as in the
case of nitrogen capacity. See FIG. 2.
O.sub.2 VSA process performance was simulated using a global energy and
mass balance model similar to one described by Smith, O. J. and
Westerberg, A. W. "The Optimal Design of Pressure Swing Adsorption
Systems", Chemical Eng. Sci. 1991, 46(12), 2967-2976, which is routinely
used as an indicator of relative performance in adsorbent screening. This
model is similar to "Flash" calculations in distillation (e.g., McCabe, W.
L. and Smith, J. C., "Unit Operations in Chemical Engineering, 3rd
edition, McGraw Hill, New York (1976), p 534).
The computer process model was used to simulate a standard O.sub.2 VSA
process cycle such as that described in G.B. 2,109,266-B that included
adsorption, purge, and desorption at chosen pressures and end-of-feed
temperature. The model is equilibrium based; i.e., it assumes no spatial
concentration gradients and complete bed utilization. Temperature changes
within the bed during the cycle are included, but the model does not
account for temperature gradients (i.e., the bed temperature is uniform at
any given time). As a first approximation, this is a reasonable assumption
in the case of equilibrium-based separation processes. Binary equilibria
are estimated using ideal adsorbed solution theory (IAST) (Meyers, A. L.
and Prausnitz, J. M. American Institute of Chemical Engineers Journal
1965, 11, 121). This theory is accepted for physical adsorption of
nitrogen-oxygen mixtures on zeolites at ambient temperatures (Miller, G.
W.; Knaebel, K. S.; Ikels, K. G. "Equilibria of Nitrogen, Oxygen, Argon,
and Air in Molecular Sieve 5A", American Institute of Chemical Engineers
Journal 1987, 33, 194). Inputs for the program include isotherm parameters
for N.sub.2 and O.sub.2, and adsorbent physical properties.
By way of placing the model in perspective, its predictions are comparable
with data from an experimental vacuum swing adsorption unit with 8 feet
long, 4 inch diameter beds. Data were compared for three different
adsorbents at a variety of operating conditions. There is excellent
agreement between pilot unit data and model predictions for Bed Size
Factor (BSF), O.sub.2 Recovery, and Actual Cubic Feet evacuated per
lb/mole Evacuation gas (ACF/Evac). These are the key parameters that
determine the product cost from any oxygen VSA plant.
Table II compares the results of the process simulations for an O.sub.2 VSA
process cycle with a feed pressure of 1000 torr, an end of feed
temperature of 75.degree. F., and an evacuation pressure of 300 torr for
lithium, sodium LSX-zeolite containing 85% lithium to a typical commercial
5A zeolite used for air separation. The Recovery, BSF, and ACF/Evac are
normalized to a value of 1.0 for the commercial 5A zeolite. The 85%
lithium, sodium LSX-zeolite of the present invention has significantly
higher Recovery and lower BSF than the commercial 5A zeolite, and only
moderately higher ACF/Evac.
TABLE II
______________________________________
O.sub.2 VSA Computer Process Simulations
sample relative relative relative
identity Recovery BSF ACF/Evac
______________________________________
commercial 5A 1.00 1.00 1.00
15% (Na, Li) LSX
1.24 0.65 1.05
______________________________________
The present invention provides a high nitrogen capacity adsorbent at lower
exchange levels of lithium than the prior art lithium X-zeolites. This is
significant because of the difficulty in achieving high levels of lithium
exchange, such as 88% and above. Therefore, the present invention provides
a unique solution to the problem of a lower cost, higher performing
adsorbent for separating nitrogen from less strongly adsorbed gas species,
which is more readily synthesized than prior art adsorbents, particularly
at comparable nitrogen capacities.
The present invention has been set forth with regard to several preferred
embodiments, but the full scope of the present invention should be
ascertained from the claims below.
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